Three modes of heat transfer are used in baking biscuits: radiation, conduction and convection.
1. Direct Gas Fired / Indirect Radiant oven 2. Conduction from steel band 3. Convection oven
All objects above a temperature of absolute zero radiate energy to their surroundings. This energy or radiation is emitted as electromagnetic waves which travel at the speed of light. The waves may travel through a vacuum or other medium. When they impact an object, they are partially absorbed and partially reflected. Good emitters are also good absorbers of thermal radiation.
Infra-red radiation is in the wavelength band of 0.7-300 microns (above visible light). Higher temperatures produce shorter wave lengths. Typical wavelengths in a radiant oven are around 4.6 – 6.4 microns, which provides good heat penetration of the dough pieces. Infrared radiation for baking is emitted principally by the DGF burner flames and by the radiant tubes in an Indirect Radiant oven.
Radiant tubes in an Indirect Oven High rate direct gas fired burners
The most important mode of heat transfer for baking is infrared radiation, which has the following advantages:
- Penetrative heat transfer: Infrared radiation penetrates biscuit doughs by approximately 4mm, (depending on wavelength and moisture content). It is the only heat transfer mode to truly bake the product from the centre. This is the key advantage of baking by infrared radiation
- Biscuit structure: because radiation penetrates the dough pieces, it is essential to achieving good structure with optimum volume and texture and is always the main mode of heat transfer in the first part of the baking process
- Even moisture content: radiant baking ensures a low moisture gradient from centre to the outer surface of the biscuit. It is the best heat transfer mode to avoid “checking” (cracks appearing in the biscuit after baking)
- Efficiency: heating of the surrounding air in the baking chamber is not necessary, which lowers energy consumption
- Colouring: radiation enables highlighted colour contrasts for crackers and rotary moulded products, whereas convection gives an overall, bland, even colour
- Versatile: infrared baking is suitable for all types of biscuit
Conduction transfers heat from the oven band directly to the base of the dough pieces. The heat transfer is dependent on the temperature and heat mass of the oven band and the surface area of the band in contact with the dough piece. With steel bands and heavy mesh bands this approximates to full contact and is very effective.
Ovens with band pre-heat can quickly transfer heat into the base of the dough pieces and achieve rapid development of the biscuit structure and texture; this is particularly valuable for cracker baking.
Convection baking uses hot air jets which impinge directly on the top of the dough pieces and the underside of the oven band. This system effectively dries and colours the surface of the dough pieces. However it produces a hard, dry skin on the dough pieces and will prevent good expansion and “lift” of the product if used at the start of the baking process. Also it is a cause of a moisture gradient between the surface (very dry) and the centre of the biscuit (more moist). This may result in “checking”, (cracking of biscuits after baking), unless the moisture gradient is reduced after baking.